Fresh milk is bactericidal, and spermicidal. I think that pasteurised milk is probably bactericidal/spermicidal too. The bactericidal effect can be nullified by heating the milk at 90C for around 10 minutes. There are references in the Dairy literature from around 1910, more from bacteriological literature around 1930, more from milk as a semen diluent literature around 1950-55 (J.Animal Sci., I think) and a Medical research Council UK paper around 1985??).
If it is a fresh milk, it may contain various naturally occurring antimicrobial compounds such as lactoperoxidase (LP system), lysozyme, lactoferrins, immunoglobulins, that inhibits the growth of microorganisms in the initial phase. If it is heat processed milk, then also may be presence of antibiotic residues may lead to increase in lag phase.